fixed : Visual build compressing stdin with multi-threading enabled fails

It was multiple reasons stacked :
- Visual use a different code path, because ZSTD_NEWAPI is not defined
- fileio.c sends `0` as `pledgedSrcSize` to mean `ZSTD_CONTENTSIZE_UNKNOWN`  (fixed)
- ZSTDMT_resetCCtx() interpreted `0` as "empty" instead of "unknown" (fixed)

programs/Makefile

@@ -37,8 +37,8 @@ endif
 
 CPPFLAGS+= -I$(ZSTDDIR) -I$(ZSTDDIR)/common -I$(ZSTDDIR)/compress \
            -I$(ZSTDDIR)/dictBuilder \
-           -DZSTD_NEWAPI \
-           -DXXH_NAMESPACE=ZSTD_   # because xxhash.o already compiled with this macro from library
+           -DXXH_NAMESPACE=ZSTD_ \
+           -DZSTD_NEWAPI
 CFLAGS  ?= -O3
 DEBUGFLAGS+=-Wall -Wextra -Wcast-qual -Wcast-align -Wshadow \
             -Wstrict-aliasing=1 -Wswitch-enum -Wdeclaration-after-statement \